Overstory-regeneration relationships in mixed-conifer forests of the southwestern United States
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The ponderosa pine, dry- and wet-mixed conifers, and spruce-fir forests are major forest types in the southwestern United States. The ponderosa pine and dry mixed-conifer systems have historically relied on fire to maintain structure and composition. With the changes in fire regimes, there has been a long-term progression toward more shade-tolerant species such as Abies concolor. Using data from the USDA Forest Service Forest Inventory and Analysis program, this study focused on the relative proportions of each species in the seedling, sapling, and overstory tree cohorts. The hypothesis is that regeneration and subsequent development by species were proportional to the species in the overstory, accounting for shade tolerance. Transitions to a different future forest combination, if that transition was occurring, was related to the time since the previous disturbance, usually fire, and the relative proportion of Abies in the overstory mix. This paper examines a subset of the relationship between overstory basal area, latitude, elevation, and seedling numbers in the understory by species. We found three species – Picea engelmannii, Pseudotsuga menziesii, and Pinus ponderosa – displayed an interaction between overstory basal area and latitude upon seedling numbers in the understory. There was little separation at the more southerly locations, suggesting an external limiting factor, soil moisture availability, overcame species-specific silvical characteristics and competitive abilities. Future work will examine the presence of particular species in the overstory and their influence upon their own and other species' regeneration.
Keywords: multiple species forests, regeneration trends, succession, microenvironment
ID: 3486970